Use new thread-local const-init

Let's see if this gives us any speedup - some of the TLS state modified in this
commit *is* pretty heavily accessed, so we can hope!

compiler/rustc_middle/src/lib.rs

@@ -50,6 +50,7 @@
 #![feature(control_flow_enum)]
 #![feature(associated_type_defaults)]
 #![feature(iter_zip)]
+#![feature(thread_local_const_init)]
 #![recursion_limit = "512"]
 
 #[macro_use]